Biological wastewater treatment process and reactor

ABSTRACT

An aerated treatment of water is performed using a serially coupled reactor and flotation tank and is performed using so much activated sludge in the water that small bubbles from an aerator are entrained in the activated sludge, leading some of or all the activated sludge to flotate in the flotation tank. A reactor ( 1 ) and a flotation tank ( 36 ) for performing the process are equipped with at least one aerator ( 19, 20, 21, 22 ). A number of preferred processes and designs elaborate on the invention, including how the invention may be incorporated into existing wastewater treatment facilities and exploited there.

TECHNICAL FIELD

The technical field of the invention is continuously or intermittentlyaerated, biological treatment of either wastewater or contaminated waterintended for use or reuse, in one or more reactors.

Some concepts will be used in the following text:

Reactor

In all the following text, the word “reactor” is to be understood in thebroad sense of the word, i.e. it is to be understood as any container ofone or more reaction processes, regardless of whether the container isopen at its top (e.g. a wastewater basin in a wastewater treatmentplant) or essentially closed (as e.g. common, industrial fermentors). Areactor may well be part of a reactor plant in which a number ofreactors and possibly other tanks are serially coupled, so that contentscan move from one reactor or tank to another reactor or tank.

Loop Reactor

Similarly, the term “loop reactor” is to be understood in its broadsense, i.e. it is to be understood as any reactor of loop type,including but not limited to the well-known loop reactors for wastewatertreatment, and loop fermentors of any kind, and may employ any meansknown in the art for propulsion of the reactor contents through theloop. Loop reactors may be of an outer type, which means that no part ofthe loop is surrounded by another part of the loop. Also, they may be ofan inner type, which, in contrast, means that some part of the loop isactually surrounded by another part of the loop. This is especially wellknown in vertical loop reactors, in which a so-called ‘downcomer’ may becompletely surrounded by the so-called ‘upcomer’ or vice versa. Innerloop reactors with a surrounding downcomer may employ one or more,internal upcomers, as may inner loop reactors with a surrounding upcomeremploy one or more, internal downcomers. Employing more than one upcomeror downcomer provides a parallel coupling of loops in the reactor.

Loop Passage

A reactor loop has an average, inner length, and the reactor contentshave an average speed of movement through the loop. When the averagespeed will have led to a movement covering the average, inner length ofthe reactor loop, one loop passage is said to have taken place. In thecase of an aerated liquid, it is the average speed of the liquid whichdefines the term “loop passage”.

Liquid-liquid Ejector

Liquid-liquid ejectors are ejectors in which a liquid is used as amotive fluid and a liquid is a suction fluid.

Activated Sludge

Activated sludge is sludge with a high content of living, microbialbiomass. It is often used in treatment of wastewater and is then addedto the wastewater as part of the treatment process. The activated sludgeis then typically harvested from the treated wastewater, so that (atleast a part of) the sludge can hereby be reused. Use of activatedsludge is one way to conduct biological water treatment.

Entrainment of Bubbles in Suspended, Particulate Matter

Bubbles are said to be entrained in suspended, particulate matter, whenthe rising speed of the bubbles in the liquid containing both thesuspended, particulate matter and the bubbles is substantially reducedby the presence of the particulate matter.

BACKGROUND ART

In a number of biological substance-turnover or substance-producingprocesses of various kinds, bubbles of air or other gas are introducedinto or emerge in a usually aqueous liquid.

In some situations, the limiting factor for the turnover or productionprocess is the removal of used bubbles.

This situation is known from the European patent EP 0 510 010, in whicha centrifugal separator built into the loop of a loop reactor is used tocarry out a complete or partial removal of the bubbles from the reactionliquid.

In other situations, the limiting factor for the turnover or productionprocess is the oxygen consumption capacity of the liquid itself.

This may be the case in conventional use of activated sludge fortreatment of water.

Treatment of water using activated sludge may be carried out usingalternating periods of aeration and no aeration. Through this,nitrification of ammonium, nitrite, and organically bound nitrogen canbe achieved in periods of aeration, whereas denitrification of nitrateand possible nitrite to gaseous nitrogen can be achieved in periods ofno aeration.

The disadvantage to such an alternation between periods of aeration andperiods of no aeration is that this—in conventional use of the activatedsludge—is a slow process, measured as degree of achieved treatment percubic meter of water per hour.

DISCLOSURE OF INVENTION

The present invention provides a teaching as to how the speed of acontinuously or intermittently aerated, biological water treatment canbe increased.

This is achieved by the process of the invention for continuously orintermittently aerated (using air with or without oxygen enrichment)treatment of water in a reactor, which process is characterized by thereactor being serially coupled to a flotation tank, and by the water,being aerated extensively in a continuous or intermittent mode,contained in the reactor and being passed into the flotation tank fromthe reactor containing so much activated sludge that small bubbles froman aerator of any suitable kind and placement are entrained in theactivated sludge in the water, leading to flotation of some of or allthe activated sludge in the flotation tank.

The present invention also concerns a reactor for performing the saidprocess, which reactor comprises means for extensive aeration of thewater. The reactor and aeration means, and the flotation tank, may be ofany kind suitable for performing the said process.

The invention is based on a discovery disclosed herein of an ability ofactivated sludge, as it is known from wastewater treatment plants, toentrain small bubbles very effectively, if only a somewhat higherconcentration than usual of the activated sludge is employed in thewastewater treatment.

Traditionally, around 4 kg of activated sludge is employed per cubicmeter of wastewater. In the process of the invention, the optimalconcentration of the activated sludge will depend on the nature of thesludge, which will in turn be dependent on the character of thewastewater and the design of the overall treatment process, but sludgeconcentrations in or around the interval of 40-100 kg activated sludgeper cubic meter of water will probably be most beneficial.

The increased amount of activated sludge per cubic meter of waterincreases the oxygen consumption capacity in the water, whereby anincreased aeration intensity can be employed and exploited, acceleratingthe treatment of the water. Similarly, the increased amount of activatedsludge per cubic meter of water accelerates the denitrificationprocesses taking place during any unaerated time periods.

These accelerations of the process rates are advantageous in that theyprovide a higher water treatment capacity per cubic meter of reactorvolume, allowing a reduction in the number of and/or the sizes ofprospective reactors and thereby providing savings in both theconstruction costs of water treatment plants and the maintenance costsof these.

It will not necessarily be all the sludge that flotates in the flotationtank, but the effectiveness of the flotation will be increaseable (andadjustable) through (adjusted) aeration into the flotation tank,preferably at a depth below the inlet for water, sludge, and bubblesfrom the loop reactor. Such aeration is best conducted by use of one ormore liquid-liquid ejectors being supplied air, as described furtherbelow.

The flotated sludge may be led away (e.g. on a ramp or a belt conveyoror through pumping) from the flotation tank's upper part.

A smaller or larger part of the thus harvested sludge may be reused inthe reactor.

In a preferred process according to the invention, the conduit leadingwater and activated sludge from the reactor to the flotation tankcomprises a liquid-liquid ejector being supplied air into it or insideit and—by the interaction between the motive liquid and the suctionliquid—committing this air into small bubbles being entrained in theactivated sludge, the ejector thus acting as an aerator according to theinvention. The ejector preferably receives, as a suction fluid, thecomplete flow of water and sludge being led directly into the conduitfrom the reactor.

The advantage to using the ejector is in that this provides a very evenand effective distribution af small bubbles into the sludge to beflotated.

In yet a preferred process according to the invention, the reactor is aloop reactor, and the aeration bubbles used in the loop reactor are sosmall, having been produced in any way known in the art of producingsmall bubbles, that the bubbles are essentially entrained in theactivated sludge in the water at at least one point of aeration andeither kept entrained during the passage of the water through at leastthe majority of the reactor loop or at least released so slowly from thewater that some of the bubbles remain in the water during the passage ofthe water through at least the majority of the reactor loop.

The advantage to entraining the bubbles in the sludge inside the loopreactor is that the hereby reduced bubble rising speed in the waterreduces the energy consumption for the aeration, measured as the amountof energy used per kg of oxygen absorped by the water.

In an outer or inner, vertical loop reactor (employing at least onedowncomer and at least one upcomer), the reduction in the energyconsumption is achieved in that the reduced rising speed of the bubblesmakes the combined density of the water, sludge, and bubbles in theupcomer less different from the density of the same in the downcomer,whereby the energy consumption for propulsion of the water through theloop is reduced.

In a horisontal loop reactor (e.g. a basin with a central wall aroundwhich the water is circulating, thereby making it an outer loopreactor), the reduction in the energy consumption is achieved throughthe retention time of the bubbles in the water being increased, wherebythe bubbles gain time to yield more oxygen to the water, before theyreach the surface of the water.

In a one more, preferred process according to the invention, the loopreactor is essentially vertical, and small bubbles entrained in theactivated sludge inside the loop reactor are being led with the waterand activated sludge passed through the conduit to the flotation tank.

The advantage to the loop reactor being vertical is in that this makesit easier to ensure that the small bubbles stay entrained in the sludge,until the water reaches the upper part of the loop reactor. This alsoenables sludge with the same concentration of entrained bubbles as inthe loop reactor to be led to the flotation tank, possibly making theejector of the first-mentioned, preferred process obsolete.

In a further, preferred process according to the invention, the water inthe loop reactor is, for each loop passage, supplied less oxygen thanwhat the activated sludge could have consumed during the loop passage.

The advantage to this is in the discovery disclosed herein that smallbubbles entrained in the sludge to a large extent are rather sedentarywithin the sludge, whereby the bubbles (when they are sufficientlylow-numbered per small volume of water) divide the water and theactivated sludge suspended in it into a spatial mosaic of oxygenated andessentially oxygen-free zones. Hereby, the nitrification anddenitrification processes in the water can take place simultaneouslywithin one and the same reactor and even with very shortdiffusion/convection paths for the nitrogen compounds involved in theprocesses.

Thus, one avoids having to separate the nitrification and thedenitrification processes either spatially (e.g. into one each of twoconnected reactors) or chronologically (as in the BioDenitro process ofthe Danish company Krüger A/S).

This accelerates the treatment of the water even further, and inaddition it simplifies both the construction of the water treatmentplant and the operation of this.

Whereas one in the BioDenitro process optimizes the interplay betweennitrification and denitrification by advancing or delaying the shiftbetween various modes of operation, usually in two connected reactors,on the basis of measurements of dissolved nitrate and ammonium in thereactor or both the reactors, the preferred process disclosed here willallow one to optimize this interplay by measuring in just one reactorand by use, quite simply, of regulation of the aeration rate in thisreactor—completely without any need for shifts between different modesof operation in the reactor.

In yet a preferred process according to the invention, bubbles areseparated out of the water and its activated sludge during passagethrough a centrifugal separator, preferably a cyclone separator, builtinto the reactor loop, enhancing the removal of used bubbles from thewater and sludge.

In a vertical reactor loop with a downcomer and an upcomer (out of whichone may be part of more than one loop), the centrifugal separator maypreferably be built in as the upper part of the upcomer.

In a further, preferred process according to the invention, thecentrifugal separator is a cyclone separator with lamellae placedhelically inside the cyclone separator, so that water and activatedsludge may move longitudinally through the cyclone separator undersimultaneous rotation by moving inside the spaces between the lamellae.

The advantage to the lamellae is that they may enhance the separation ofthe bubbles from the water and the sludge at high concentrations of theactivated sludge.

In still a preferred process according to the invention, the aeration ofthe water and its activated sludge is performed by inlet of air into orinside one or more liquid-liquid ejectors in the loop reactor. Theejector or ejectors thus act as an aerator or aerators according to theinvention.

The advantage to this use of liquid-liquid ejectors is that it allowsfor large-scale production of bubbles small enough to be entrained inthe sludge.

This preferred process is especially advantageous if all or essentiallyall the water is led through the ejector or ejectors for every looppassage, so that the ejector or the ejectors are effectively part of thereactor loop. This spreads the bubbles very effectively out into all thesludge.

In the case of a vertical reactor loop with a downcomer and an upcomer(out of which the one part can be common to more than one loop), theejector or ejectors may preferentially constitute the upper connectionbetween the upcomer and the downcomer.

In a further, preferred process according to the invention, water with areduced content of or completely without activated sludge is passed outof the flotation tank and into a liquid pump pumping the water to any ofor all the above-mentioned liquid-liquid ejectors as a motive fluid,possibly together with water led directly from the reactor to the liquidpump.

The advantage to this preferred process is partly in that it has turnedout that a reduced concentration of activated sludge in the motive fluidof the ejector or ejectors may improve the quality of the bubbleformation in the ejector or ejectors, and partly in that the reducedconcentration of the activated sludge better protects the microorganismsin this against damage caused by the liquid-pump.

In yet a preferred process according to the invention, water with areduced content of activated sludge is passed out of the flotation tankand into a continuously or intermittently aerated, succeeding watertreatment tank, i.e. a succeeding water treatment reactor.

The advantage to this preferred process is that it allows for a morecomplete treatment of the water without the activated sludge beingnourished insufficiently in the first-mentioned reactor, i.e. thereactor passing water and sludge into the flotation tank.

In still a preferred process according to the invention, water with areduced content of activated sludge is conducted out of the flotationtank and into a sludge precipitation tank, from which a part of theprecipitated sludge may possibly be led or transported back into thefirst mentioned reactor, i.e. the reactor passing water and sludge intothe flotation tank.

This precipitation will be necessary for the removal of the remainingsludge from the water, if not all sludge has been removed by flotation(and possibly also some precipitation) in the flotation tank andpossible passing of water from the flotation tank to a liquid pumpand/or a succeeding water treatment tank.

In any case, water will have to be led out of the flotation tank, if oneis applied, and is there any activated sludge left in this water, thenthis sludge should preferably be precipitated out of the water in orderfor the water to have been properly treated.

The two last-mentioned, preferred processes according to the inventionmay, by the way, be combined such that water with a reduced content ofsludge is led through the succeeding water treatment tank to the sludgeprecipitation tank.

Hereby, these two tanks will have worked roughly in the same manner as acomplete water treatment plant of usual design.

Are the two tanks already constructed, then the invention will havesupplemented a traditional water treatment plant with an extra reactorwith an increased water treatment rapidity and possibly alsosimultaneous nitrification and denitrification and reduced energyconsumption per kg of oxygen absorped by the water. This will be aninexpensive, both in investment terms and in operating costs, increasein the overall capacity of an already existing water treatment plant.

In a further, preferred process according to the invention, the reactorpassing water into the flotation tank is a loop reactor (as in other ofthe above, preferred processes), and new water to be treated is led intoan unaerated or continuously or intermittently aerated, preceeding watertreatment tank or reactor, and from this onwards into a liquid pumppumping the water to the loop reactor's liquid-liquid ejector orejectors as a motive fluid.

The advantages to such a preceeding water treatment tank are well knownand include conversion of phosporous compounds (when the preceedingwater treatment tank is unaerated) and stripping of any dissolved andtoxic gases and/or volatile liquids (when the preceeding water treatmenttank is aerated).

In addition, a preceeding water treatment tank without directthroughflow of water to the loop reactor can be utilized as a buffertank, so that variations in the inflow of new water to be treated can besmoothed out, providing a more even inflow of water into the loopreactor. That the preceeding water treatment tank in the preferredprocess according to the invention is capable of acting as such a buffertank is due to the fact that the onwards flow of the water from thepreceeding water treatment tank takes place through the water pump andthe ejector/ejectors rather than by direct throughflow from the tank tothe loop reactor.

If the concentration of activated sludge in the preceeding watertreatment tank is either zero or lower than in the loop reactor, thenthe further advantages to using other water than water from the loopreactor itself in the liquid pump are as described earlier in thepresentation of preferred processes.

The above, preferred process can be combined with the preferred processof passing of water from the flotation tank to the liquid pump.

Hereby, a control option is created, in that the proportion between howmuch water is flowing from the preceeding water treatment tank into thepump and how much water is flowing from the flotation tank into the pumpwill be adjustable.

If the water from the preceeding water treatment tank has a relativelyhigh concentration of those substances, which are to be removed to adesired degree from the water, a hereto correspondingly reduced flow ofwater from the preceeding water treatment tank may be employed, so thatthe supply rate of these substances to the loop reactor is kept at adesired and not too high level.

Conversely, if the water from the preceeding water treatment tank has alower concentration of the substances, then a faster flow of water fromthe preceeding water treatment tank may be employed, so that the supplyrate of the substances to the loop reactor is kept at a desired levelalso in this situation.

Ideally, the proportion between how much water is flowing from thepreceeding water treatment tank to the pump and how much water isflowing from the flotation tank to the pump will be subjected to on-linecontrol based on measurement results from substance concentration metersmeasuring the quality of the water in the loop reactor, with theexception that a low water level in the preceeding water treatment tankshould lead to a reduction in the flow of water from this tank to thepump, whereas a high water level in the preceeding water treatment tankshould lead to an increase in the flow of water from this tank to thepump. The water level in the preceeding water treatment tank may bemeasured in any suitable way known in the art.

If a liquid-liquid ejector is employed as an aerator in the flotationtank or in the conduit leading to it from the loop reactor, then themotive fluid being used in this ejector should preferably be water fromonly the loop reactor and/or the flotation tank itself and not from thepreceeding water treatment tank. Hereby, water from the preceeding watertreatment plant has not been led past the loop reactor. Hence, all waterfrom the preceeding water treatment tank will be treated in the loopreactor.

Is also one or more liquid-liquid ejectors employed as aerator/aeratorsin the loop reactor, then it will be necessary to use two liquid pumpsfor the motive waters to the ejectors, so that water from the preceedingwater treatment tank is not mixed into the water from the loop reactorand/or the flotation tank being used as motive fluid in the ejector inthe flotation tank or in the conduit leading to it from the loopreactor.

In one more, preferred process according to the invention, part of andonly part of the activated sludge being reused in the loop reactor issupplied to this by being added to the water in the preceeding watertreatment reactor and being transferred through the liquid pump and intothe liquid-liquid ejector or liquid-liquid ejectors in the loop reactor.

The activated sludge being added to the water in the preceeding watertreatment reactor may be harvested from the flotation tank, if one isemployed, or from the precipitation tank, if one is employed, or fromboth of these.

The usual practice, which may actually be employed in the presentinvention as well, when reusing activated sludge harvested at the lastin a series of tanks in a water treatment plant is to pass all thereused sludge back to the first (reactor) tank or tanks (if there aremore than one in a parallel setting) in the series. In the processaccording to the present invention it is, however, when the passing ofwater and sludge from the first tank (the preceeding water treatmentreactor) takes place through the liquid-liquid ejector or theliquid-liquid ejectors of the loop reactor, better to divide theactivated sludge to be reused, such that only a part of the sludge isadded to the water in the first tank, whilst the rest of the sludge isadded directly to the water in the loop reactor.

Is e.g. 20 kg of activated sludge employed per cubic meter of water inthe preceeding water treatment reactor, this will, however, increase thewater treatment capacity of this approximately fivefold as compared to apreceeding water treatment reactor with only the usual 4 kg of activatedsludge per cubic meter of water.

Hereby, the preceeding water treatment reactor will, at a plantredesigned to make use of the present invention, have its watertreatment capacity increased sufficiently to make good use of the watertreatment capacity increase obtained in the rest of the plant throughemployment of the present invention.

The preferred processes described above are to be understood as beingcombinable. That is, a given water treatment facility may employ anycombination of the preferred processes consistent with the wording inthe description of each preferred process and may even employ all ofthem. Any combination may be established in just one each of any reactoror tank mentioned above and being part of the combination or in aplurality of any of or all of such reactors or tanks.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be further described in more detail with reference tothe drawings, wherein:

FIG. 1 is a longitudinal, sectional view executed through the axis of anembodiment of an inner loop reactor according to the invention,

FIG. 2 is a cross-sectional view of an embodiment of an inner loopreactor according to the invention, taken along the line A-A in FIG. 1,

FIG. 3 is a cross-sectional view of an embodiment of an inner loopreactor according to the invention, taken along the line B-B in FIG. 1,

FIG. 4 is a longitudinal, sectional view executed through the axis of anembodiment of a flotation tank useable according to the invention,

FIG. 5 shows a longitudinal section through the axis of an embodiment ofa liquid-liquid ejector of a design useable in loop reactors accordingto the invention,

FIG. 6 is a cross-sectional view of the liquid-liquid ejector, takenalong the line C-C in FIG. 5,

FIG. 7 is a flow chart of an embodiment of a water treatment plantincluding a loop reactor according to the invention.

The embodiment of an inner loop reactor 1 according to the inventiondepicted in FIG. 1 in an axially longitudinal, sectional view has anouter container wall 2 and an adjacent bottom wall 3 which togetherdefine a cylindrical reaction container (i.e. reactor), which is open atits top. An inner, cylindrical tube 4 defines an inner upcomer 5 in thereactor 1, i.e. water, sludge, and bubbles move essentially upwards inthe upcomer 5. The cylindrical-annular space 6 surrounding the innerupcomer 5 is a downcomer in the loop reactor 1, i.e. water, sludge, andbubbles move essentially downwards in the downcomer 6. The outerdowncomer 6 and the inner upcomer 5 are connected below 7 to each otherat the free end of the inner tube 4. The upper end of the inner tube 4is designed to be a cyclone separator 8. The cyclone separator 8 has aninner, cylindrical core 9, around which water, sludge, and bubblesrotate while continuing to move upwards (i.e. the movement is helical)during the separation of bubbles from the water and sludge. Beneath thecyclone separator 8, the inner core 9 is tapered by a frusto-conicalextension 10.

In its lower end, the cyclone separator 8 is initiated by four helicallyarranged guiding plates 11, 12, 13, and 14 which direct water, sludge,and bubbles through four rectangular inlet apertures 15, 16 (not visiblein FIG. 1), 17, and 18 (not shown in FIG. 1) leading into the cycloneseparator 8. Hereby, the water, sludge, and bubbles are being given arotational velocity component on their way into the cyclone separator 8.In FIG. 1, the introductory edge (in the cutting surface of FIG. 1) andthe upper surface (behind the cutting surface of FIG. 1) of the guidingplate 11, the lower surface (behind the cutting surface of FIG. 1) andthe terminating edge (in the cutting surface of FIG. 1) of the guidingplate 12, and (in the cutting surface of FIG. 1) the introductory edgeof the guiding plate 13 and the terminating edge of the guiding plate 14are visible. The inlet aperture 15 is constituted by the rectangulararea defined by the introductory edge of the guiding plate 11, theterminating edge of the guiding plate 14, the inner tube 4, and the core9, whereas the inlet aperture 17 is constituted by the ractangular areadefined by the introductory edge of the guiding plate 13, theterminating edge of the guiding plate 12, the inner tube 4, and the core9. Correspondingly, the inlet aperture 16 not visible in the Figure isconstituted by the rectangular area defined by the introductory edge ofthe guiding plate 12, the terminating edge of the guiding plate 11, theinner tube 4, and the core 9, whilst the inlet aperture 18 not shown inthe Figure is constituted by the rectangular area defined by theintroductory edge of the guiding plate 14, the terminating edge of theguiding plate 13, the inner tube 4, and the core 9.

If there is a desire to have the cyclone separator 8 equipped withlamellae, as described above for one preferred process, then theselamellae may be devised similarly to the guiding plates 11, 12, 13, and14 with the difference that their extension inwards from the inside ofthe inner tube 4 should not extend all the way to the core 9 of thecyclone separator 8.

At the upper end of the cyclone separator 8, four air-suction capableliquid-liquid ejectors 19, 20, 21, and 22 (not shown in FIG. 1) aremounted. The tubings leading motive fluid and air to the liquid-liquidejectors are not shown; reference is made to FIGS. 5 and 6. Theliquid-liquid ejectors 19, 20, 21, and 22 constitute the upperconnection for water, sludge, and bubbles between the upcomer 5 anddowncomer 6 of the loop reactor 1. In FIG. 1, the suction inlet 23 forliquid of the ejector 19 is seen in the cutting surface of the Figure,and behind the cutting surface of the Figure a further part of theejector 19 is seen, the ejector 20 is seen behind the cutting surface ofthe Figure, and the suction inlet 25 for liquid of the ejector 21 isseen in the cutting surface of the Figure. The rest of the ejector 21 isin front of the cutting surface of the Figure, as is all of the ejector22.

At its top, the cyclone separator 8 is delimited by a frusto-conicaltapering 31 leading to a vertical tube 32 for withdrawal of bubblesand/or used air liberated from bubbles.

An inlet member for activated sludge, consisting of a tube 33 extendingdown into the downcomer 6 of the loop reactor 1 and on top of this tube33 a funnel 34, is shown behind the cutting surface of the Figure.

The loop reactor 1 is to be understood as having a size consistent withit being used in a water treatment plant.

During operation of the loop reactor 1, the liquid-liquid ejectors 19,20, 21, and 22 are supplied motive fluid, possibly from one or moreliquid pumps, and air sucked in, preferably through a throttle valvecontrolling the aeration rate. The ejectors 19, 20, 21, and 22 pumpwater and sludge from the upcomer 5 of the loop reactor 1 to thedowncomer 6 of the loop reactor 1. In addition to this, the ejectors 19,20, 21, and 22 communicate the air being sucked in by them into suitablysmall bubbles distributed essentially evenly into the through theejectors 19, 20, 21, and 22 flowing water and sludge.

Also during the operation of the loop reactor 1, some water with sludgeand bubbles is passed out of the loop reactor 1 through an outlet member35 having its inlet in the downcomer 6.

It is not crucially important for the operation of the loop reactor thatthe cross-sectional shape of the upcomer 5 is circular beneath thecyclone separator 8. Other cross-sectional shapes, e.g. polygonal,inclusive of quadratic and rectangular, are also usable. Similarly, thecross-sectional area of the upcomer 5 does not have to be the samebeneath the cyclone separator 8 as inside the cyclone separator 8.

Nor does the shape of the downcomer 6 of the loop reactor 1 have to becylindrical-annular; its outer wall 2 does not even have to becylindrical. The outer wall 2 may have other cross-sectional shapes,e.g. polygonal, inclusive of quadratic and rectangular. Furthermore, theloop reactor can be made wider by the employment of more than one innertube 4 equipped with everything required for its operation, includingthe cyclone separator 8 and ejectors 19, 20, 21, and 22. A plurality ofsuch tubes 4 fully equipped can be placed at the same depth in but atdifferent locations within a downcomer 6 wide enough for this and willthen constitute a parallel coupling of upcomers 5 within one commentdowncomer 6.

Also, the number of the ejectors 19, 20, 21, and 22 is not fixed. Theredoes not have to be exactly four ejectors leading from the cycloneseparator 8—or each cyclone separator 8 in case of employment of morethan one inner tube 4. The number of ejectors from one cyclone separator8 may be less than four, equal to four, or higher than four.

Nor do the ejectors 19, 20, 21, and 22 (if four of these are employed)have to be directed exactly horizontally outwards from the separator 8.The direction may well be obliquely upwards, so that the ejectors passwater, sludge, and bubbles not only outwards but also upwards from thecyclone separator 8. Obliquely downwards will probably be lessadvantageous.

Furthermore, the outlet member 35 does not have to have its inlet in thedowncomer 6. The inlet may be positioned anywhere in the loop reactorbut in the centrifugal separators and the ejectors.

FIG. 2 shows a cross-sectional view taken along the line A-A in the loopreactor shown in FIG. 1. 2 Denotes the outer wall of the loop reactor (1in FIG. 1), 4 denotes the inner tube (the section is taken through thecyclone separator (8 in FIG. 1)), and 9 denotes the core in the cycloneseparator. Beneath the cutting surface of FIG. 2, the upper surfaces ofthe guiding plates 11, 12, 13, and 14 are visible, as are the positionsof the inlet apertures 15, 16, 17, and 18, being bounded, as the inletsare, by the visible, terminating edges of the guiding plates and theintroductory edges of these, as each of the introductory edges in thisembodiment happens to be positioned directly underneath the terminatingedge of one of the other guiding plates. In the cutting surface of theFigure, sections through the liquid-liquid ejectors 19, 20, 21, and 22and their respective suction inlet apertures 23, 24, 25, and 25 forliquid (these suction inlet apertures are in the Figure coincidentallyplaced directly above part of one each of the inlet apertures 15, 16,17, and 18) as well as the ejector outlet apertures 27, 28, 29, and 30are visible. 33 Denotes the inlet tube for activated sludge, and 35denotes the outlet member for water, sludge and bubbles from thedowncomer 6.

As in FIG. 1, the tubings leading motive fluid and air to theliquid-liquid ejectors are not shown; reference is made to FIGS. 5 and6.

The disposition of the liquid-liquid ejectors 19, 20, 21, and 22 impartsa weak rotational movement to the water, sludge and bubbles at the topof the downcomer 6 of the loop reactor (1 in FIG. 1). This distributesthe activated sludge from the inlet tube 33, in that this activatedsludge moves with the surrounding water and sludge around the inner tube4 and thereby becomes sufficiently dispersed to allow for it to beeffectively mixed into the water, sludge and bubbles coming from theliquid-liquid ejectors 19, 20, 21, and 22.

FIG. 3 shows a cross-sectional view taken along the line B-B in the loopreactor shown in FIG. 1. 2 Denotes the outer wall of the loop reactor (1in FIG. 1), 4 denotes the inner tube (the section is taken beneath thecyclone separator (8 in FIG. 1)), and 35 denotes the outlet member forwater, sludge and bubbles from the downcomer 6.

FIG. 4 shows an axially longitudinal section through an embodiment of aflotation tank 36 for use according to the invention. The flotation tank36 has an outer wall 37 and an adjacent bottom wall 61 as well as aninlet member 35 (which equals the outlet member 35 in FIGS. 1-3) forwater, sludge, and bubbles from the loop reactor (1 in FIG. 1). Inaddition to this, the flotation tank 36 has an outlet member 38 forwater either without activated sludge or with a reduced concentration ofactivated sludge and an additional outlet member 39 for water eitherwithout activated sludge or with a reduced concentration of activatedsludge and for use as a motive fluid in the ejectors (19, 20, 21, and 22in FIGS. 1-2).

Preferentially, the outlet member 38 is provided with a throttle valveor any other regulatory device known in the art for regulation of theflow rate through the outlet member 38. This provides a control of theamount of water present in the loop reactor (1 in FIG. 1) at any givenrate of water flow into the loop reactor (1 in FIG. 1).

Furthermore, one or more aerators (not shown in the Figure), preferablyone or more air-supplied liquid-liquid ejectors of a design more or lesssimilar to the design of the ejectors (19, 20, 21, and 22 in FIGS. 1-2)in the loop reactor (1 in FIG. 1) but possibly smaller than these, canbe commissioned in the lower part of the flotation tank, preferablybelow the inlet member 35. This will enhance the flotation of theactivated sludge and thereby reduce the amount of activated sludgemoving from the inlet member 35 to the outlet members 38 and 39.

The flotation tank 36 has been drawn slightly higher than the loopreactor (1 in FIG. 1) as an adaptation to the possibility of the uppersludge surface in the flotation tank being above the water surface inthe loop reactor (1 in FIG. 1) due to bubbles trapped in the sludge inthe flotation tank 36.

FIG. 5 shows an axially longitudinal section through an embodiment of aliquid-liquid ejector for use in a loop reactor according to theinvention. Only the cutting surface itself is shown in the Figure.

Preferably, the liquid-liquid ejectors in the loop reactor (1 in FIG. 1)are of identical design. The Figure shows, as an instance, the ejector20. 40 Denotes a tubular inlet member for air being sucked in, which airis introduced into the liquid-liquid ejector 20 through the inletaperture 55. 41 Denotes a tubular inlet member for motive fluid (waterand usually also some activated sludge—the motive fluid usually comesfrom a liquid pump but may alternatively be derived from a containerwith a higher water pressure at the level of the ejectors than in theloop reactor (1 in FIG. 1)). The tubular member 41 is terminated by anozzle aperture 42. In the Figure, the tubular member 41 is shown asbeing longer than the tubular member 40. This is only in the drawing;both the two members will typically extend further than shown in theFigure. The ejector 20 has a suction inlet aperture 24 for water andsludge from the cyclone separator (8 in FIG. 1). In the Figure, theejector 20 is shown with a funnel-shaped inlet section 43, whichpreferably joins the tubular mid-section 44 of the ejector smoothly, andwhich makes the cross-sectional area of the mid-section 44 smaller thanthe cross-sectional area of the suction inlet aperture 24. Such afunnel-shaped inlet section 43 may be advantageous by adapting theejector 20 to a higher internal flow velocity than the flow velocity inthe cyclone separator (8 in FIG. 1). The tubular mid-section 44 of theejector 20 contains the mixing zone in the ejector. The ejector outletaperture 28 of the ejector 20 has a larger cross-sectional area than thesuction inlet aperture 24 and is positioned at the end of a preferablysmoothly designed, gradually widening outlet section 45 which serves thepurpose of braking the velocity of the water, sludge and bubbles priorto their exit out into the downcomer (6 in FIG. 1) of the loop reactor(1 in FIG. 1), thereby enhancing the ejector's 20 pumping efficiency.

FIG. 6 shows a cross-sectional view of the ejector in FIG. 5, takenalong the line C-C in FIG. 5. Behind the cutting surface, the outersurface of the outlet section 45, the inner surface of the inlet section43, and the noble aperture 42 are visible. In the cutting surface, thesuction inlet aperture 24, the tubular air inlet member 40 and thetubular motive fluid inlet member 41 are visible.

FIG. 7 shows a flow sheet of an embodiment of a water treatment plantincluding a loop reactor according to the invention. 46 Denotes apreceeding water treatment reactor, and 47 denotes a succeeding watertreatment reactor surrounding the loop reactor 1 and a flotation tank36. 35 Denotes the conduit leading water, sludge, and bubbles from theloop reactor 1 to the flotation tank 36, 38 denotes a conduit oraperture for water and sludge from the flotation tank 36 to thesucceeding water treatment reactor 47, and 39 denotes an outlet forwater, possibly with some sludge in it, from the flotation tank 36 andfor use as a motive fluid in ejectors. 48 Denotes a liquid pumpsupplying motive fluid to the ejector or ejectors (19, 20, 21, and 22 inFIGS. 1-2) of the loop reactor 1. The pump 48 receives water, possiblywith some sludge in it, from the flotation tank 36 through a conduit 56and also water with sludge from the preceeding water treatment reactor46 through a conduit 57. 49 denotes a further liquid pump being suppliedwater, possibly with some sludge in it, from only the flotation tank 36and to be used as a the motive fluid in one or more air-suppliedejectors in the lower part of the flotation tank 36. 50 Denotes aconduit passing water and sludge from the succeeding water treatmentreactor 47 to a precipitation tank 51. Some harvested sludge is passedto the loop reactor 1 and the preceeding water treatment reactor 46 bythe conduit or transporter (of any suitable kind known in the art) 52.It is possible to use two separate conduits or transporters 52, so thatthe harvested sludge passed to the preceeding water treatment reactor 46can be of a composition differing from the composition of the harvestedsludge passed to the loop reactor 1—e.g. sludge only from the flotationtank or sludge only from the precipitation tank or just anotherproportion between these than in the sludge passed to the loop reactor1. Water to be treated in the plant is passed into the preceeding watertreatment tank 46 through a conduit 53, and treated water is passed outof the precipitation tank through a conduit 54.

Valves of any kind known in the art may be provided in the conduits 56and/or 57 for control of the flows of water from the preceeding watertreatment reactor 46 and the flotation tank 36, respectively, to thewater pump 48.

In the Figure, withdrawal 60 of surplus sludge is shown to take placefrom both the flotation tank and the precipitation tank. If less sludgeis to be removed, then the removal may be arranged to be from only theprecipitation tank.

58 Denotes a conduit passing air to the ejectors (19, 20, 21, and 22 inFIGS. 1-2) in the loop reactor 1. Preferably, a valve of any kind knownin the art for control of the air flow into the loop reactor 1 isprovided in the conduit 58.

59 Denotes a conduit passing air to one or more liquid-liquid ejectorsin the lower part of the flotation tank 36. The deep position of theejector or ejectors will make inclusion of an air pump in the conduit 59advantageous. If an air pump is provided in the conduit 59, then theaeration rate in the flotation tank 36 will be adjustable throughadjustment of the pumping work performed by the air pump.

For further illustration and explanation of the invention, the followingshall be mentioned:

At the commencement of operations at a water treatment plant includingboth a loop reactor according to the invention and of further tanks atleast the flotation tank and a succeeding water treatment reactor and aprecipitation tank, initiation of the biological water treatment may beperformed as a propagation of the activated sludge mainly in thesucceeding water treatment reactor, with sludge being harvested in theprecipitation tank and return of the harvested sludge to the succeedingwater treatment reactor. During this phase, water may be led directlyinto the succeeding water treatment reactor, bypassing the loop reactorand flotation tank.

Before the concentration of the activated sludge passes the upper limitfor effective precipitation in the precipitation tank, one beginspassing sludge to the loop reactor.

During the initial passing of sludge to the loop reactor, it will beadvantageous to close or at least limit the flow of water into the loopreactor, so that there will be either no flow at all or at least only avery limited flow of activated sludge out of the loop reactor, while theconcentration of the activated sludge in the loop reactor is beingincreased.

When the concentration of the activated sludge in the loop reactor issufficiently high, one may close any water flow bypassing the loopreactor, so that all subsequent water flow to the succeeding watertreatment reactor takes place through the loop reactor and flotationtank. Similarly, one may at this point cease any passing of sludgedirectly into the succeeding water treatment reactor.

If there, during the commencement of operations, is also a preceedingwater treatment reactor in use (as in e.g. the well-known BioDeniphoprocess of Krüger A/S), then all said above for commencement ofoperations will still be valid. Preferably, the preceeding watertreatment reactor should be in operation right from the start, receivingall water inflow to the plant and passing this on to either thesucceeding water treatment reactor or the loop reactor, as describedabove for commencement of operations in a plant without the preceedingwater treatment reactor. If the loop reactor is being bypassedinitially, then part of or all the harvested sludge may be passed to thepreceeding water treatment reactor and through this to the succeedingwater treatment reactor.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention, at least when using theinvention for treatment of waste water, is considered to be to use allthe preferred processes described above, at least if very highconcentrations of activated sludge are employed in the loop reactor. Theuse of lamellae in the cyclone separator may be omitted by usingslightly lower concentrations of the activated sludge in the water, andthe use of an aerator in the flotation tank and/or in the conduitleading water into this is only relevant in case of otherwiseinsufficient flotation of the activated sludge.

In addition to the embodiments of the invention as defined in theclaims, the following embodiments are also preferred:

-   I. A process wherein air bubbles are being introduced into the    flotation tank by an aerator placed inside the flotation tank,    preferably beneath an inlet aperture for water from the reactor    passing water and sludge into the flotation tank.-   II. A process wherein activated sludge is being passed from the    upper end of the flotation tank and into the reactor passing water    and sludge into the flotation tank.-   III. A process wherein water either devoid of activated sludge or    with a reduced concentration of activated sludge is passed out of    the flotation tank through an outlet aperture in the lower part of    the flotation tank and through a conduit including a liquid pump    onwards to one or more liquid-liquid ejectors in the reactor and/or    the flotation tank and/or the conduit connecting the reactor to the    flotation tank, said water acting as a motive fluid in said ejector    or ejectors.-   IV. A process wherein water with a reduced concentration of    activated sludge is passed out of the flotation tank through a    conduit or aperture leading the water into a continuously or    intermittently aerated, succeeding water treatment reactor.-   V. A process wherein water with a reduced concentration of activated    sludge is passed out of the flotation tank through a conduit which    leads the water, possibly through the succeeding water treatment    reactor mentioned in the above process IV, into a precipitation    tank, from which part of or all the precipitated sludge is possibly    passed to the reactor passing water into the flotation tank.-   VI. A process wherein the reactor passing water into the flotation    tank is a loop reactor, and water, possibly containing activated    sludge, from an aerated or unaerated, preceeding water treatment    tank or reactor is led through a conduit including a liquid pump and    onwards to one or more liquid-liquid ejectors in the loop reactor,    said water acting as a motive fluid in said ejector or ejectors.-   VIII. A process wherein a part of the activated sludge being passed    to the loop reactor (e.g. as in the above processes II, V, or VI) is    being passed via the preceeding water treatment reactor, so that the    sludge is suspended in the water in the preceeding water treatment    reactor and passed with this through the conduit and the liquid pump    to the liquid-liquid ejector or the liquid-liquid ejectors in the    loop reactor, whilst the remaining part of the activated sludge    being passed into the loop reactor is passed directly into this.

1. A process for effecting a continuously or intermittently aeratedtreatment of water in a loop reactor system, said process comprising thesteps of: providing a loop reactor comprising: a propulsive means fordriving the circulation of water, sludge, and bubbles through a reactorloop; at least one small-bubble aerator capable of producing bubblessmall enough to be essentially entrained in said sludge; and means forgrowing or supplying activated sludge in an amount leading to aconcentration of activated sludge in said water in said loop reactorgreater than 4 kg of activated sludge per cubic meter of water beingtreated and high enough to facilitate entrainment of said bubbles fromsaid small-bubble aerator in said sludge; and growing or supplying saidactivated sludge in an amount leading to a higher concentration ofactivated sludge in said water in said loop reactor greater than 4 kg ofactivated sludge per cubic meter of water being treated and high enoughto facilitate entrainment of bubbles from said small-bubble aerator,said bubbles thereby becoming essentially entrained in said activatedsludge in said water in said loop reactor.
 2. The process according toclaim 1, wherein said small-bubble aerator is a liquid-liquid ejectorbeing supplied air by introduction of this into its at least one throat,so that the air is being comminuted into said bubbles by the interactionbetween a motive liquid and a suction liquid inside said ejector.
 3. Theprocess according to claim 2 further comprising the step of passingessentially all of said water and its activated sludge through saidejector or a plurality of such ejectors for every loop passage, so thatsaid ejector or plurality of ejectors effectively constitute part ofsaid reactor loop.
 4. The process according to claim 3 furthercomprising the step of generating a driving force provided by saidejector or plurality of ejectors for propelling said water with itssludge and bubbles through said reactor loop.
 5. The process accordingto any of claim 4 further comprising the step of passing said reactorwater, for each loop passage, through a centrifugal separator built intosaid loop reactor for separation of said bubbles from said water.
 6. Theprocess according to any of claim 5 further comprising the step ofleading said separated bubbles either to the surface of said water orout of said loop reactor.
 7. The process according to any of claim 6,wherein said centrifugal separator is a cyclone separator.
 8. Theprocess according to claim 6 further comprising the step of introducingair into a peripheral outlet conduit for water and sludge from saidcentrifugal separator.
 9. The process according to any of claim 8further comprising the step of coupling in series said loop reactor to aflotation tank, and that said small bubbles entrained in said activatedsludge inside said loop reactor pass with said water and activatedsludge through said conduit to said flotation tank, leading to flotationof some of or all said activated sludge in said flotation tank.
 10. Theprocess according to any of claim 9 further comprising the step ofsupplying less oxygen to said water, for each loop passage, than couldhave been utilized by said activated sludge in said water during theloop passage.
 11. A loop reactor system for effecting a continuously orintermittently aerated treatment of water, said loop reactor systemcomprising: at least one downcomer and at least one upcomer beinginterconnected at a lower end of said upcomer and at an upper end ofsaid upcomer, thereby constituting a reactor loop; a propulsive meansfor driving the circulation of water, sludge, and bubbles through saidreactor loop; at least one small-bubble aerator capable of producingbubbles small enough to be essentially entrained in said sludge; meansfor supplying activated sludge in an amount leading to a concentrationof activated sludge in said water in said loop reactor greater than 4 kgof activated sludge per cubic meter of water being treated and highenough to facilitate entrainment of said bubbles from said small-bubbleaerator in said sludge; and a centrifugal separator incorporated intothe upper part of said upcomer.
 12. The loop reactor system according toclaim 11, wherein said centrifugal separator is a cyclone separatorintegrated in such a way into the upper half of said upcomer thatessentially all said water passing up through said upcomer passesthrough said cyclone separator on its way up inside said upcomer. 13.The loop reactor system according to claim 12, wherein said cycloneseparator is integrated into the upper quarter of side upcomer.
 14. Theloop reactor system according to claim 12, wherein said aerator is aliquid-liquid ejector comprising an air inlet aperture positionedadjacent an inlet aperture for an ejector's motive fluid, terminating ina gradually widening outlet section, and constituting an upperconnection between said upcomer and said downcomer.
 15. The loop reactorsystem according to claim 14, wherein said ejector further comprises anoutlet aperture being completely submerged in said water in saiddowncomer.
 16. The loop reactor system according to claim 11 furthercomprising a flotation tank having a liquid-liquid ejector beingsupplied air into it and by the interaction between a motive liquid anda suction liquid committing this air into small bubbles.
 17. The loopreactor system according to claim 16 further comprising a conduitleading said water and activated sludge from said loop reactor to saidflotation tank.
 18. The loop reactor system according to claim 17,wherein said ejector is built into said conduit so that it receives, asa suction fluid to said ejector, the complete flow of water and sludgebeing led directly into said conduit from said loop reactor.
 19. A loopreactor system comprising: a loop reactor comprising: at least onedowncomer and at least one upcomer being interconnected at a lower endof said upcomer and at an upper end of said upcomer, therebyconstituting a reactor loop; a propulsive means for driving thecirculation of water, sludge, and bubbles through said reactor loop; atleast one small-bubble aerator capable of producing bubbles small enoughto be essentially entrained in said sludge; means for supplyingactivated sludge in an amount leading to a higher concentration ofactivated sludge in said water in said loop reactor greater than 4 kg ofactivated sludge per cubic meter of water being treated and high enoughto facilitate entrainment of said bubbles from said small-bubble aeratorin said sludge; and a centrifugal separator incorporated into said upperpart of said upcomer; a flotation tank having a liquid-liquid ejectorbeing supplied air into it and by the interaction between a motiveliquid and a suction liquid committing this air into small bubbles; anda conduit leading said water and activated sludge from said loop reactorto said flotation tank; wherein said ejector is built into said conduitso that it receives, as a suction fluid to said ejector, the completeflow of water and sludge being led directly into said conduit from saidloop reactor.